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Apps are sending your personal data to online trackers in violation of Apple's privacy rules. That's the conclusion of a Wall Street Journal investigation, which determined that a number of popular apps - such as Pandora and MySpace - transmit device and demographic data to advertisers without first asking the user for permission.

The Wall Street Journal tested 101 apps for iPhone and Android and found that 56 of them transmitted unique device identifiers (UDIDs) to a flock of advertisers without asking or even informing the app user. 47 of the evaluated apps send location data and five (including Pandora) transmit your personal details such as age and gender. Apple says that they vet apps before approving them on the App Store, but the Journal found that at least one - Pumpkin Maker - had managed to skirt the rules.

These concerns have been raised before, and one important thing to be aware of is that the collected data is usually processed in batches without any specific information about your identity. However, the fact that UDIDs are being sent out presents the very real danger that someone - be it a black-hat hacker or government organization - could be intercepting this information, slowly building a database of everyone's personal details. Meghan O'Holleran of Traffic Marketplace notes that "you can't clear a UDID like you can a cookie." She says "that's how we track everything."

A mobile-ads executive interviewed by the Journal responded by asserting that "in the world of mobile, there is no anonymity." Michael Becker of the Mobile Marketing Association says that the mobile phone is "always with us. It's always on."